PROPHYLAXIS OF BOVINE TRICHOSTRONGYLIDOSIS IN THE ALPINE REGION - EFFECT OF PASTURE CONTAMINATION ON INFECTIONS IN CALVES RECEIVING A MORANTEL SUSTAINED-RELEASE TRILAMINATE BOLUS IN MID-JULY

In the alpine region, mixed grazing systems with cattle of different a ge, origin and prophylaxis against trichostrongylid infections are mos t common. Under these conditions the administration of anthelmintic de vices to susceptible calves is frequently postponed to June or July to achieve a better protection during the period of increased pasture in fectivity in summer and autumn. In a field experiment with 27 first-ye ar grazing calves a morantel sustained-release trilaminate bolus (MSRT , Pfizer) was given to two groups (Groups A and B) of nine naturally i nfected calves each, on 22 July. Calves of Group B were moved to a cle an pasture (B) 1 day later, whereas the calves of Group A remained on the previous pasture (A) together with nine untreated calves (Group C) . The contamination with infective larvae (L3) on Pasture A remained b elow 1000 L3 kg-1 dry matter, which was sufficient to produce clinical parasitic gastroenteritis in five of nine control calves. The MSRT bo lus reduced the mean egg output by more than 90% within 14 days after administration and prevented clinical parasitic gastroenteritis in the calves of Groups A and B. Owing to the persisting infection risk on P asture A, the mean serum pepsinogen levels reached about 3000 mU tyros ine in the calves of Group A in September compared with approximately 2000 mU in Group B grazing the clean pasture. However, the differences in pasture contamination were not reflected in the mean bodyweight of the calves, which were 20 kg heavier at the end of the trial in both MSRT-treated groups compared with the control calves (P < 0.01). As th ere is a high probability that a moderate larval contamination is pres ent on prealpine and alpine community pastures in summer, the metaphyl actic use of an MSRT bolus in mid-July is likely to limit trichostrong ylid infections within a subclinical range and thus provides sufficien t protection of susceptible calves against parasitic gastroenteritis.